Oxygen Diffusion In Oxides Research Articles

Investigation on the oxygen reduction reaction mechanism of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ@La2NiO4+δ core-shell structure cathode for solid oxide fuel cells

Excellent electrochemical performance is achieved by PrBa0.5Sr0.5Co1.5Fe0.5O5+δ@La2NiO4+δ (PBSCF@LN) core-shell structure cathode, LN coating's existence enhances the cathode catalytic activity by promoting the oxygen reduction reaction (ORR) surface process. Besides the LN's excellent oxygen adsorption-desorption property, roughness surface of LN coating provides more ORR active sites compared to pristine PBSCF cathode. Thorough investigation is carried out based on the PBSCF and LN oxide's oxygen diffusion mechanism and corresponding ORR process elementary steps are constructed. Combine the theoretical derivations with experimental results, rate-determining step of PBSCF and PBSCF@LN cathode can be ascertained. With LN coating's introduction, PBSCF cathode's rate-determining step transfers from surface oxygen exchange to oxygen species' crossing LN coating, reducing thickness of LN coating can further improve PBSCF@LN cathode’ performance.

High temperature oxidation behaviors of Ni3Al-bonded cermets

Abstract High temperature oxidation behaviors of Ni 3 Al/Ni-bonded cermets were studied in this paper and results revealed that mass gain of Ni 3 Al-bonded cermet is less than that of Ni-bonded cermet at different oxidation temperatures and the mass gain is in accord with oxidation equation (Δm/S) = k n ·t 0.6 , in which oxidation rate constant, k n , at 800 °C, 900 °C and 1000 °C is 0.157, 0.912 and 2.27 respectively. High temperature oxidation kinetic of Ni 3 Al-bonded cermet belongs to quasi-parabolic kinetic behavior. Moreover, oxidation mechanisms and oxidation morphology of Ni 3 Al-bonded cermet were also investigated. During high-temperature oxidation, Ni 3 Al binder would be oxidized, forming double oxide, NiAl 2 O 4 , which can restrain the diffusion of oxygen in oxide, resulting in an improvement of oxidation resistance of cermet. The cross section morphology of oxidized Ni 3 Al-bonded cermet is comprised of oxide layer (OL), transformed layer (TL), and substrate. Obviously, some pores can be observed in OL and also slight pores exist in TL. Thickness of OL of Ni 3 Al-bonded cermet increased with temperature increasing and keeping time prolonging. In oxidation process, Al, Ti etc. would diffuse outwards to OL from inside of cermet and O 2 would diffuse inwards into cermet forming oxygen permeable zone, namely transitional layer.

Isotopic exchange and diffusion of oxygen in oxides with different bulk and subsurface diffusivities

A theoretical model has been proposed to describe the kinetics of oxygen isotopic exchange between gas and oxides with subsurface regions different from the bulk in the rate of oxygen transfer (surface phases, grain boundaries, etc.). Exact solutions valid for an arbitrary ratio between the exchange and diffusion rates have been obtained for both the time dependence of the total 18 O tracer concentration and the dependences of 16 O 16 O, 18 O 16 O, 18 O 18 O molecules concentrations in a gas. The latter makes it possible to analyse the contributions to the kinetics from various types of exchange taking diffusion into account. The results obtained also allow the determination of the exchange rate and diffusivity from the experiment with high enough accuracy, as shown by the example of the oxide YBa 2 Cu 3 O 6+ x .

Oxygen diffusion in olivine: Effect of oxygen fugacity and implications for creep

Oxygen self‐diffusion experiments on single crystals of San Carlos olivine (∼Fo92) at 1200° ≤ T ≤ 1400°C, oxygen fugacities (ƒO2) along the Ni‐NiO and Fe‐FeO buffers, and silica activity at the olivine‐orthopyroxene buffer yielded results that follow the relationship D = 2.6 × 10−10 ƒO20.21±0.03 exp [−266±11 (kJ mol−1/RT)], where D is the diffusion coefficient in m2 s−1 and ƒO2 is given in pascals. The activation energy compares reasonably well with results for pure forsterite. The positive dependence of ƒO2 implies that the oxygen defect responsible for diffusion is an interstitial rather than a more sterically reasonable oxygen vacancy. Diffusion of oxygen in other close‐packed oxides has also shown a positive dependence on ƒO2. The rate of creep of single‐crystal olivine at fixed orthopyroxene activity also shows a positive ƒO2 dependence. If oxygen interstitials should be shown to be unimportant in oxygen diffusion in oxides, then coupled mechanisms such as countervacancy diffusion must be appealed to in order to explain the positive ƒO2 dependence. Such processes are rate‐limited by the diffusion of metal vacancies which also display a positive ƒO2 dependence in olivine. Compared with data for silicon diffusion in forsterite, our data indicate that oxygen is not the slowest diffusing species in olivine. The activation energy for oxygen diffusion is also low compared to that obtained in a majority of measurements of creep in single‐crystal olivine. Hence if oxygen diffusion contributes to the control of creep in olivine, it must be coupled to another process having a nonzero activation energy. A subinvestigation of the rate of sublimation from polished olivine surfaces, using a high‐resolution thermal balance, showed surface erosion rates at 1300°C of 0.13±0.10 nm/h. Such values are far too slow to noticeably affect our diffusion measurements at temperatures ≤1400°C.